As is well known, automatic transmissions for vehicles include a manual shift lever, which is indirectly actuated by the operator of the vehicle to select from a plurality of predetermined operating modes, such as Park, Reverse, Neutral, Over-drive, Drive, and Low. The manual lever is connected to a manual valve disposed within a valve body in the transmission. The manual lever actuates a manual valve plate that initiates hydraulic manual valve port changes in the manual valve corresponding to the operating mode selected by the operator, thereby engaging the gear ratio selected.
When the operator of the vehicle selects the "Park" mode, a park lock mechanism is engaged to prevent rotation of the output shaft of the transmission. Generally, the park lock mechanism is operated by reciprocating a pawl actuator rod, which is pivotally mounted to the manual lever. The pawl actuator rod typically includes a generally conical end opposite the pivot connection end. The pawl actuator rod, thus, drives the conical end against a park pawl. The pawl is mounted on a pivot shaft attached to the transmission housing. A pawl tooth of the park pawl then meshes with a park gear. The park gear is fixed to the rotatable transmission output shaft to rotate therewith. Engagement of the park pawl with the park gear thereby prevents rotation of the transmission output shaft.
In operation, when the "Park" mode is selected, the manual lever rotates in a first direction to linearly translate the pawl actuator rod until the conical end of the pawl actuator rod engages with the park pawl. Most often, the park gear will not be in alignment with the park pawl tooth when the driver selects "Park." For this reason, the conical end of the pawl actuator rod is spring loaded, such that the driver can fully select "Park," and actual "Park" will automatically occur as soon as the vehicle rolls slightly forward or backward, allowing the park pawl to engage between the teeth of the park gear. This engagement between the park pawl and the park gear prevents rotation of the transmission output shaft. This spring-loaded conical end of the pawl actuator rod also avoids the catastrophic engagement of "Park" if a driver should inadvertently select "Park" while travelling at speed, since the dynamics of the system will cause the park pawl to ratchet (or bounce) across the gear teeth above 3-5 mph. Since this spring-engaged mechanism requires significant linear stroke, it must thereby be actuated by a relatively long lever-arm from the manual lever. When a mode other than "Park" is selected, the manual lever rotates in a second direction to linearly translate the pawl actuator rod until the conical end of the pawl actuator rod disengages from the park pawl. Since the park pawl is spring biased to disengage from the park gear even when there is no tendency for the vehicle to roll in either direction, disengagement of the conical end of the pawl actuator rod from the park pawl causes the park pawl to disengage from the park gear to enable free rotation of the transmission output shaft.
It should be appreciated that an operator must be able to disengage "Park" even when the vehicle has a strong tendency to roll, such as on a steep grade or with the tires compressed against a curb. Therefore, the park pawl tooth-form and park gear teeth-forms must have pressure angles (both forward and reverse) to assure self-ejection. These high self-ejection forces in turn generate a very high clamping load on the cylindrical portion of the conical end of the park pawl actuator rod. This high clamping load creates a high friction force to resist pulling the park pawl actuator, and its cylindrical portion, out from under the park pawl.
Thus, it can be appreciated that the prior art park lock mechanism includes numerous disadvantages. Specifically, the pawl actuator rod is pivotally coupled to the manual lever at a predetermined distance from the rotation axis of the manual lever. This arrangement thereby defines a single stroke length of the manual lever to engage or disengage the park pawl. However, it should be appreciated by one skilled in the art that to engage the park pawl in a "Park" position, the force applied to the pawl actuator rod by the operator must overcome the spring force of the rod spring acting on the conical end and the spring force of the park pawl spring acting on the park pawl. Preferably, a long stroke length of the manual lever is used to compress these springs to minimize the effort required by the operator. However, when disengaging the park pawl from the "Park" position, when a strong rolling load exists, a high force is required. Thus, a short stroke length of the manual lever is preferred to disengage the park pawl from the "Park" position to minimize the effort required by the operator. Once initial friction is overcome (motion started), the full ejection should preferably follow.
Accordingly, there exists a need in the relevant art to provide a park lock mechanism for an automatic transmission that is capable of varying the stroke length of the manual lever depending upon whether the park pawl is being engaged or disengaged from the "Park" position. Furthermore, there exists a need in the relevant art to provide a manual lever in a park lock mechanism that requires less force to disengage a pawl actuator rod from a park pawl than prior practice has provided.